This invention relates to improvements in vital wheel detectors for railways and, in particular, to a detector apparatus which injects an oscillating electrical current into one rail of the track and does not rely on the sensing of a shunt current to detect the presence of a wheel in a detection zone.
Wheel detectors are employed as key components of various control systems used in railways, including grade crossing warning control systems, hotbox detectors, and control systems utilized in hump yards. A "vital" wheel detector, in contrast to non-vital, must unfailingly detect the presence or passing of a car wheel and fail in a safe mode, i.e., disclose a failure so that the control system in which it functions can produce an appropriate warning. To be truly vital, such a detector upon failure either fails to produce an output signal or responds in the same manner as if a wheel were present in the detection zone.
Vital requirements should not be limited to electrical failures of the detector circuitry or components. A mechanical or physical fault should also produce a failure indication. Typically, a wheel detector is secured to or mounted adjacent the track and thus a dismounted condition or separation of its parts should cause a loss of or change in the output of the detector indicative of its physical disability. Furthermore, it is desired that a vital detector not depend upon rail/wheel shunting to detect the approach or presence of a train because of the uncertainty, under rusty rail conditions, of relying upon the establishment of an electrical shunt across the rails by the wheels and axles of the train.